I need a WP8 project and an Android project. Until now I have only seen a PCL called Core which is linked to all platform projects. The problem is, that I have platform specific references, which I need to include, depending on the platform. How can I handle this?

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With PCL projects, you would typically create an interface, then create separate platform specific implementations.

So you may have an ILocationWatcher in your PCL core project. Then you can create a WinPhone.LocationWatcher and Droid.LocationWatcher in each of the platform projects.

Usually you would use some sort of Inversion of Control (IoC) container to register the patform specific type at startup. Then your Core project would get an instance, either via dependency injection or resolving via the IoC container. Your core PCL would program strictly against the interface and not have to worry about the specific implementation.

This is how MvvmCross plugins work. Pretty much all the plugins come with a core interface and a platform specific implementation.